An electric discharge machine will be described which is one of the aforementioned variety of machining units.
FIG. 1 is an explanatory diagram showing the arrangement of an application control apparatus for an electric discharge machine which has been disclosed by Japanese Patent Application Publication No. 10769/1987 for instance. In FIG. 1, reference numeral 1 designates a machining electrode; 2, a workpiece to be machined; 3, a machining vessel; 4, a machining solution; 5, a spindle; 6, a drive motor; 7, a speed or position detector; 20, a machining unit; 21, an electrode position control section; 22, a machining power source; 23, a state recognizing section; and 31, an application control section. In this description, the term "machining unit" is intended to mean that which includes the above-described parts 1 through 7, and 21 and 22.
The operation of the application control apparatus thus constructed will be described. First, the operator determines initial machining conditions taking into account the material and size of a workpiece to be machined, machining quantity, finish accuracy, and so forth, and sets them for the machining unit 20. For instance, the operator sets the peak, pulse width and pulse interval of a pulse current, an electrode pull-up period, an electrode pull-up distance, electrode servo parameters, etc.
After the initial machining conditions have been set, a discharge machining operation is started. That is, the machining power source 22 applies the pulse voltage across the inter-electrode space (or discharge gap) between the machining electrode 1 and the workpiece 2 to induce electric discharges, whereby the workpiece 2 is machined with the electrode 1 which is moved relative to the workpiece 2. The electrode position control section 21 compares an average inter-electrode voltage provided by the state recognizing section 23 with a reference voltage, to control the position or speed of the machining electrode 1 thereby to maintain a suitable distance between the machining electrode 1 and the workpiece 2.
In an electric discharge machining operation, the distance between the machining electrode 1 and the workpiece 2 (hereinafter referred to as "an inter-electrode space or discharge gap", when applicable) is generally small, ten microns to several tens of microns. Therefore, in the case where the machining area is large, it is relatively difficult for the waste material such as sludge formed during discharge machining to flow through the inter-electrode space. As a result, an abnormal electric discharge is liable to occur. That is, the waste material stays in the inter-electrode space, so that electric discharges are inducted collectively at the position of the waster material. This difficulty attributes to the fact that, during electric discharge machining, waste material such as sludge is formed more than removed. The difficulty may be eliminated by the following method: The abnormal condition is detected or predicted, to suppress the production of waste material, or to accelerate the removal of waste material.
FIG. 2 shows the variations in position of the machining electrode 1. More specifically, the part (a) of FIG. 2 shows the variation in position of the machining electrode in the case where a normal electric discharge machining operation is carried out; whereas the part (b) of FIG. 2 shows the variation in position of the machining electrode in the case where the abnormal condition occurs in the inter-electrode space. During electric discharge machining, the machining electrode 1 is vibrated with an amplitude of 10 to 100 microns. In a normal electric discharge machining operation, the point 101 where the downward movement of the machining electrode is changed to the upward movement (hereinafter referred to "a minimum point 101", when applicable) is moved downwardly gradually as the discharge machining operation advances; whereas when the abnormal condition occurs in the inter-electrode space, the minimum point 101 is moved upwardly. Therefore, upon detection of the upward movement of the minimum point 101, by decreasing the pulse width of the pulse current supplied by the machining power source 22, the formation of waste material such as sludge in the inter-electrode space can be suppressed; and by increasing the periodic electrode pull-up distance, the removal of waste material from the inter-electrode space can be accelerated.
In FIG. 1, the state recognizing section 23 detects the minimum point 101 from the variation in position of the machining electrode 1, and informs the application control section 31 of the upward or downward movement of the minimum point 101. When the upward movement of the minimum point 101 exceeds a predetermined threshold value, the application control section 31 determines that the abnormal condition has occurred in the inter-electrode space, and applies an instructions to the electrode position control section 21 to increase the electrode pull-up distance to accelerate the removal of the waste material or to the mechining power source 22 to decrease the pulse width of the pulse current to suppress the formation of waste material.
FIG. 3 is a circuit diagram of the application control section. The application control section applies an instruction 111 to the electrode position control section 21 to increase the electrode pull-up distance when the level of the minimum point 110 detected by the state recognizing section 23 exceeds a predetermined threshold value.
Another example of the conventional machining unit application control apparatus will be described.
FIG. 4 is a block diagram showing the conventional machining unit application control apparatus which has been disclosed by Japanese Patent Application (OPI) No. 297017/1986 (the term "OPI" as used herein means an "unexamined published application") for instance. In FIG. 4, reference numeral 51 designates a discharge machining process including an electric discharge phenomenon; 52, the amount of state of the discharge machining process; 53, an electrode control system; 54, an inter-electrode distance between a machining electrode and a workpiece which is controlled by the electrode control system; 55, a state detector for detecting the amount of state; 56, a detection value provided by the state detector 55; 57, an instruction value setting unit for setting the state of the discharge machining process; 58, an instruction value provided by the instruction value setting unit 57; 59, a difference value obtained from the instruction value 58 and the detection value; 60, a jump controlling unit for controlling a jumping operation; 61, an amount of jump operation; 62, a switching unit fort selecting an inter-electrode distance control according to the difference value 59 or a jumping operation according to the amount of jump; 63, an amount of operation which the switching unit 62 applies to the electrode control system 53; 64, a machining electrode position signal; 65, a jump setting unit for setting an amount of jump or a period of jump according to a machining depth in advance in order to perform a suitable jumping operation; and 66, a jump instruction value which the jump setting unit 65 applies to the jump controlling unit according to the machining electrode position signal 64.
In FIG. 1 the mechanical part of the machining unit is represented by the parts (1) through (7), whereas in FIG. 4 it is represented as objects to be controlled which are an inter-electrode distance inputted and an amount of machining state outputted.
An inter-electrode distance control operation will be described with reference to FIG. 4. In FIG. 4, the difference value 59 is obtained from the instruction value 58 provided by the instruction value setting unit 57 which sets a desirable state for the discharge machining process and the detection value 56 provided by the state detecting unit 55 which detects the state of the discharge machining process. The difference value 59 thus obtained is applied through the switching unit 62, as the amount of operation 63, to the electrode control system 53. The electrode control system 53 operates to adjust the inter-electrode distance 54 so that the difference value 59 be zeroed. Thus, desirable discharge machining conditions are maintained at all times.
However, as the discharge machining operation advances, the waste material formed is caused to stay in the inter-electrode gap between the machining electrode and the workpiece, as a result of which short-circuit occurs in the inter-electrode gap frequently. Hence, it is difficult to maintain the electric discharge machining operation stable merely by the above-described inter-electrode distance control.
Therefore, in general, a pumping action attributing to the jumping operation of the machining electrode is utilized to remove the waste material from the inter-electrode gap between the machining electrode and the workpiece.
The term "jumping operation" as used herein is intended to mean the periodic operation that, during inter-electrode gap control, the machining electrode is forcibly pulled up a predetermined distance from its machining position irrespective of the instruction value 58 or the detection value 56 and is then returned to the original machining position.
The jumping operation of the machining electrode is controlled as follows: Jumping conditions such as an amount of jump which is an electrode pull-up distance determined according to a machining depth and a period of jump which is an electrode pull-up period are set for the jump setting unit 65 in advance. A machining depth is obtained from the position signal 64 of the machining electrode which is in operation, and the jump instruction value 66 is transmitted to the jump controlling unit 60 referring to the jumping conditions set in the jump setting unit 65, as a result of which the jump controlling unit 60 applies the amount of jump operation 61 through the switching unit 62, as the amount of operation 63, to the electrode control system 53.
As is apparent from the above description, the machining electrode jumping operation is essential for maintaining the electric discharge machining operation stable at all times; however, in view of machining efficiency, it can be said that the jumping operation does not contribute directly to the machining of the workpiece. Thus, in order to improve the machining efficiency, it is essential to perform the jumping operation of the machining electrode most suitably.
In order to realize the most suitable jumping operation of the machining electrode, it is necessary to determine the jumping conditions such as an amount of jump and a period of jump not only from a machining depth but also a machining electric power source's pulse conditions, a machining electrode configuration, the materials of a machining electrode and a workpiece, and so forth. Thus, in general, the jumping operation is carried out by a person skilled in the art. That is, such a skilled person monitors a discharge machining operation, to change the jumping operation suitably according to the degree of instability of the discharge machining operation.
The conventional machining unit application control apparatus is constructed as described above. Therefore, the change of the electrode pull-up distance is determined merely from the result which is provided according to the method in which the electrode pull-up distance is increased when the amount of rise of the minimum point exceeds a predetermined threshold value. Therefore, it is difficult to realize the jumping control according to an intricate method such as a method expressed vaguely by the skilled person. This is a first problem accompanying the conventional machining unit application control apparatus.
The conventional machining unit application control apparatus is organized as described above. Therefore, in order to add another method of controlling an electrode pull-up distance or to change the method, it is necessary to change the hardware realizing the method. And in the case where the method is realized by software, the software for determining the electrode pull-up distance according to the method must be modified in its entirety. It is impossible to readily add or change the know how possessed by the manufacturer or user. Furthermore, in order to allow a plurality of machining units to hold a variety of know how in common, it is necessary to allow them to have in common not only the method but also hardware or software for realizing the method. Satisfying this requirement takes time and labor. This is a second problem accompanying the conventional machining unit application control apparatus.
The conventional machining unit application control apparatus thus constructed suffers from the following difficulties: In setting jumping conditions according to the method provided by the person skilled in the art to perform a most suitable jumping operation, it is difficult to suitably express as jumping conditions the qualitative and value expression included in the method. In order to automatically change the jumping operation according to the degree of instability of the discharge machining operation (without the skilled person) it is difficult to correctly describe the standard of decision on which the skilled person determines the degree of instability. Thus, it is rather difficult to improve the discharge machining efficiency. This is a third problem accompanying the conventional application control apparatus.
In the conventional application control unit thus organized, it is difficult to modify an operator's machining method. In addition, in collecting an operator's machining method, it is necessary to reveal the machining conditions for which the operator starts operations and the operations done by him. This is a fourth problem accompanying the conventional apparatus.
FIG. 5 is an explanatory diagram showing the arrangement of another example of the conventional electric discharge machine. In FIG. 5, reference numeral 1 designates a machining electrode; 2, a workpiece to be machined; 3, a machining vessel; 4, a machining solution; 5, a Z-axis; 6, a drive motor; 7, a speed and position detector; 8 and 9, an X-axis and a Y-axis, respectively; 10 and 11, an X-axis drive motor, and a Y-axis drive motor, respectively; 12 and 13, speed and position detectors for the X-axis drive motor and Y-axis drive motor, respectively; 21, an electrode position control section; 22, a machining electric power source; 23, a detection value processing section corresponding to the state recognizing section in FIG. 1; 31, an application control section comprising a numerical control unit (hereinafter referred to merely as "an NC unit", when applicable); 32, a CRT and a keyboard; 32, an I/O unit such as a paper tape reader.
The operation of the machine thus organized will be described. An automatic positioning operation is carried out as follows: The NC unit 31 applies an instruction to the machining electric power source unit 22 so that the latter outputs a DC low voltage different from that which is used for discharge machining; and the NC unit applies an instruction to the electrode position control section 21 so that the latter 21 operates to move the electrode in a specified direction along a specified axis. When the contact of the electrode with the workpiece 2 is detected by the detection value processing section 23, the NC unit 31 suspends the application of the instructions to the machining electric power source unit 22 and the electrode position control section 21. Thus, the automatic positioning operation has been accomplished.
The automatic positioning function is one of the fundamental functions of the NC unit 31. The operator determines the relative position of the electrode 1 and the workpiece 2, or measures the displacement of the electrode from the center by using the automatic positioning function in combination. The determination of the relative position of the electrode and the workpiece and the measurement of the displacement of the electrode from the center is carried out according to a positioning procedure which is considered best through the past experience of the operation, because the positioning procedure cannot be determined univocally depending on the configurations and reference values of the electrode and the workpiece. Furthermore, whether or not the result of the automatic positioning operation carried out by the NC unit 31 is acceptable is determined according to the past experience of the operator, the average value of the results of a plurality of automatic positioning operations, the deepest value in a plurality of automatic positioning operations, and the same value obtained continuously in several automatic positioning operations.
In the case where the electrode 1 and the workpiece 2 are equal in configuration and in reference surface, the positioning procedure is, in general, programmed by NC program for execution. On the other hand, it cannot be determined by the operator whether or not the result of the automatic positioning operation is acceptable; that is, the determination is carried out by utilizing the automatic positioning function of the NC unit. Therefore, if, in the automatic operation, the reference surface is smudged by some external disturbance during positioning or measuring, it is impossible to obtain the result of the positioning or measuring operation with high accuracy. In the positioning procedure, the automatic positioning feed speed and frequency belong to the know how of the operator.
The conventional automatic positioning control apparatus thus constructed suffers from the following difficulties: The operator must specify the positioning procedure for the NC unit, and he cannot be determine whether or not the result of the automatic position operation is acceptable; that is, the determination is carried out by utilizing the automatic positioning function of the NC unit. The operator's know how of the electrode positioning and measuring method is not reflected onto the automatic operation. This is a fifth problem accompanying the conventional apparatus.
Heretofore, in an electric discharge machining operation, the electrode and the workpiece are moved relative to each other in such a manner that the former is pushed into the latter, and the distance between the electrode and the workpiece in that direction is maintained constant by servo technique. Furthermore, in order to perform both a rough machining operation and a finish machining operation with one electrode, the electrode or the workpiece are moved in a direction perpendicular to the ordinary direction of feed; i.e., swinging motion is given to the electrode or workpiece.
A control method for the movement in the direction in which the electrode is pushed into the workpiece has been disclosed by Japanese Patent Application Publication No's. 19371/1986, 19372/1986, 19373/1986, 19374/1986, and 58256/1986 for instance.
With respect to the swinging motion, the control method for the movement in the direction in which the electrode is pushed into the workpiece will be described. In a first example of the control method, the electrode is swung a predetermined number of times when it reaches a desired position in the direction in which the electrode is pushed into the workpiece (hereinafter referred to as "an electrode pushing direction", when applicable), and then it is moved in the electrode pushing direction. In a second example of the control method, after the lapse of a predetermined period of time from the time instant that the electrode reaches the above-described desired position is detected with the difference between the discharge machining voltage and the reference voltage in a predetermined range, the electrode is moved in the electrode pushing direction. In a third example of the control method, after the lapse of a predetermined period of time from the time instant that the electrode reaches the above-described desired position is detected with the distance for which the electrode is moved back and forth by inter-electrode voltage serve being in a predetermined range.
In the above-described first, second and third control methods, the movement of the electrode is utilized for decision of the accomplishment of the machining operation; more specifically, it is used to determine whether or not, with the electrode reached the desired position, the workpiece has been machined uniformly to desired dimensions.
In the conventional discharge machining operation, the movement of the electrode in the direction in which the electrode is pushed into the workpiece is controlled as described above. Therefore, even when machining circumferences or machining environments are changed by variations of various factors such as the area and configuration of an electrode, machining depth, the configuration of revolution, machining conditions, the presence or absence of a jet stream of machining solution, the decision of the accomplishment of the machining operation is carried out in the same manner, as a result of which the machining accuracy is not uniform. For instance in the case where the jet stream of machining solution is used, the waster material such as sludge formed during machining is removed with high efficiency, and therefore the distance between the electrode and the workpiece may be relatively small. On the other hand, in the case where the jet stream of machining solution is not utilized, the ability of removing the waste material from the inter-electrode gap is low, and secondary electric discharge occurs through the waste material, thus increasing the inter-electrode gap between the electrode and the workpiece. Hence, if, when a plurality of workpieces are machined the same depth, for all the workpieces thus machined the accomplishment of the machining operation is decided in the same manner, then the workpiece machined without the jet stream of machining solution is relatively large in dimension; whereas the workpiece machined with the jet stream of machining solution is relatively small in dimension. This is a sixth problem accompanying the conventional apparatus.